U.S. patent number 5,557,150 [Application Number 08/429,605] was granted by the patent office on 1996-09-17 for overmolded semiconductor package.
This patent grant is currently assigned to LSI Logic Corporation. Invention is credited to Chok J. Chia, Patrick Variot.
United States Patent |
5,557,150 |
Variot , et al. |
September 17, 1996 |
Overmolded semiconductor package
Abstract
A technique for providing partially and fully overmolded
semiconductor packages is described which prevents delamination
(detachment) of the molding compound from the substrate by allowing
the molding compound to flow through holes in the substrate and
forming it into rivet-like anchors on the opposite side of the
substrate. Various shapes of rivet-like anchors are described.
Different embodiments provide for the formation of molded standoffs
and locating pins integral to the anchor structures.
Inventors: |
Variot; Patrick (San Jose,
CA), Chia; Chok J. (Campbell, CA) |
Assignee: |
LSI Logic Corporation
(Milpitas, CA)
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Family
ID: |
27502439 |
Appl.
No.: |
08/429,605 |
Filed: |
April 27, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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331263 |
Oct 28, 1994 |
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969862 |
Oct 28, 1992 |
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917894 |
Jul 21, 1992 |
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834182 |
Feb 7, 1992 |
5262927 |
Nov 16, 1993 |
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Current U.S.
Class: |
257/787; 257/666;
257/E23.032; 257/E23.043; 257/E23.066; 257/E23.07; 257/E23.125;
257/E23.179; 257/E29.022 |
Current CPC
Class: |
G03F
7/70433 (20130101); H01L 23/3121 (20130101); H01L
23/49517 (20130101); H01L 23/49541 (20130101); H01L
23/49838 (20130101); H01L 23/49861 (20130101); H01L
23/544 (20130101); H01L 29/0657 (20130101); H01L
2223/54473 (20130101); H01L 2224/48091 (20130101); H01L
2224/48227 (20130101); H01L 2224/49109 (20130101); H01L
2224/73265 (20130101); H01L 2924/01078 (20130101); H01L
2924/15151 (20130101); H01L 2924/15162 (20130101); H01L
2924/15311 (20130101); H01L 2924/15312 (20130101); H01L
2924/18301 (20130101); H01L 2224/48091 (20130101); H01L
2924/00014 (20130101); H01L 24/48 (20130101); H01L
24/49 (20130101); H01L 2224/49109 (20130101); H01L
2224/48227 (20130101); H01L 2924/00 (20130101); H01L
2924/00014 (20130101); H01L 2224/05599 (20130101); H01L
2924/00014 (20130101); H01L 2924/00014 (20130101); H01L
2224/45099 (20130101); H01L 2924/14 (20130101); H01L
2924/14 (20130101); H01L 2924/00 (20130101); H01L
2924/181 (20130101); H01L 2924/181 (20130101); H01L
2924/00012 (20130101); H01L 2924/1815 (20130101) |
Current International
Class: |
G03F
7/20 (20060101); H01L 23/544 (20060101); H01L
23/31 (20060101); H01L 23/28 (20060101); H01L
23/495 (20060101); H01L 23/498 (20060101); H01L
23/48 (20060101); H01L 023/28 () |
Field of
Search: |
;257/787,666,668 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1-106449 |
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Apr 1989 |
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JP |
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1-191457 |
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Aug 1989 |
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JP |
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2-205055 |
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Aug 1990 |
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JP |
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3-116856 |
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May 1991 |
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JP |
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Primary Examiner: Crane; Sara W.
Assistant Examiner: Clark; S. V.
Attorney, Agent or Firm: Katz & Cotton, LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of commonly-owned U.S. patent
application Ser. No. 08/331,263, filed Oct. 28, 1994; which was a
file wrapper continuation of U.S. patent application Ser. No.
07/969,862, filed Oct. 28, 1992 (now abandoned); which was a
continuation-in-part of U.S. patent application Ser. No.
07/917,894, filed Jul. 21, 1992 (now abandoned), and a
continuation-in-part of U.S. patent application Ser. No.
07/834,182, filed Feb. 7, 1992 (now U.S. Pat. No. 5,262,927, issued
Nov. 16, 1993.
Claims
What is claimed is:
1. An overmolded semiconductor package, comprising:
a sandwich structure of a leadframe disposed between an upper
substrate and a lower substrate;
a semiconductor die mounted within an opening in the lower
substrate and connected to wiring traces disposed on the lower
substrate;
means for selectively electrically connecting the wiring traces to
leads of the leadframe;
at least one hole extending through the upper and lower substrates;
and
a plastic molded structure disposed over the die and partially over
the lower substrate, and extending through the at least one hole of
the upper and lower substrates.
2. An overmolded semiconductor package according to claim 1,
wherein:
at least a portion of the plastic molded structure includes
locating pins arranged in a pattern; and
the pattern of the locating pins is adapted to enforce a particular
orientation of the package when inserted into a set of
corresponding holes formed in a printed circuit board.
3. An overmolded semiconductor package according to claim 1,
wherein the molded structure includes a rivet-like anchor.
4. An overmolded semiconductor package according to claim 1,
wherein the molded structure includes a standoff.
5. An overmolded semiconductor package according to claim 1,
wherein the molded structure includes a locating pin.
6. An overmolded semiconductor package according to claim 1,
wherein the molded structure includes a standoff with locating pin
extending therefrom .
Description
TECHNICAL FIELD OF THE INVENTION
The invention relates to the packaging of integrated circuit (IC)
semiconductor devices (chips), especially to high pin count
packages formed by injection or transfer molding.
BACKGROUND OF THE INVENTION
In order to handle and connect a semiconductor die (integrated
circuit device) to external systems, it is generally necessary to
package the die. This usually involves mounting the die to some
sort of substrate, leadframe or carrier, connecting bond pads on
the die to some sort of conductive lines or traces and forming a
package body around the die. The conductive lines or traces exit
the package body, and usually terminate in external leads or
pins.
For example, ceramic packages have a package body with a central
opening (cavity) in one face for receiving the die, and lead
fingers embedded in the body and extending into the opening. The
die is connected (usually wire bonded) to the exposed (in the
opening) portions of the lead fingers. The lead fingers are
connected (internally in the package) to pins exiting a planar
surface of the package. These pins are typically arranged in a
rectangular (e.g., square) array. This type of packaging is
expensive, and involves several elements for the ceramic package,
not the least of which is the pins. In some instances, the
die-receiving cavity is "up", on one face of the package body, and
the pins are on the other, opposite face of the package body. In
other instances, the die-receiving cavity is "down", on the same
face of the package as the pins (in which case there are no pins in
the area of the cavity). (The pins are deemed to be on the "bottom"
of the ceramic body.)
In another example, the die is attached and connected (usually wire
bonded) to a relatively rigid (thick) lead frame having several
conductive lines, and a plastic body is molded around the die and
inner ends of the conductive lead frame lines. Outer ends of the
lead frame lines exit the plastic body on two or four sides. The
leads may be closely spaced and delicate, demanding extreme care in
handling to avoid lead skew and the like. Co-planarity (more
correctly, lack thereof) is a problem with this, or any type of
leaded (having external leads) package.
In another example, the die is attached to a relatively flexible
(thin) lead frame supported by a plastic tape, and is encapsulated
in epoxy or the like. Outer ends of the lead frame lines exit the
epoxy body. These leads are even more fragile than those of plastic
packages. Both wire bonding and tape-automated bonding of the die
to the lead frame lines are well known in tape packages.
Another packaging technique is mounting the die directly to a
printed circuit board (PCB; also referred to as PWB, or printed
wiring board) substrate. The die is connected (typically wire
bonded) to one end of conductive traces on a face (surface) of the
PCB. Another end of the trace is connected to a pin, a separate
external lead, or the like.
A recent development in semiconductor packaging is exemplified by
Motorola's "OMPAC", or Overmolded Plastic Pad Array Carrier,
whereby a semiconductor die is mounted face-up in a central area of
a printed circuit board (PCB). The PCB is larger than the die. The
top surface of the PCB is provided with a number of conductive
traces that extend from near the periphery of the PCB to the
central area. The die has bond pads on its face. Bond wires extend
from these bond pads to inner ends of the traces. Near the
periphery of the PCB, there are plated (conductive) through-holes
(vias) extending from the back side of the PCB, through the PCB to
a respective trace. The back side of the PCB is provided with a
number of conductive traces, each having an end connected with a
respective via. In this manner, signals (and power) to and from the
die are connected through the bond wires, through the top side
traces, through the vias, to the bottom side traces. A plastic
molded body is formed over the die, and partially covers the top
surface of the PCB.
Such "partial" coverage of the PCB by the molding compound results
in a package body that can delaminate (separate) from the substrate
(PCB) when the molded body shrinks after molding. Consequently,
moisture can penetrate onto the chip along the delamination
interface (plane between molded body and PCB) and cause reliability
failures. The adhesion of the partially molded plastic body on to
the substrate is generally considered as the weak link in this type
of package, and is especially so when the plastic molded body gets
large and the substrate is thin and flexible.
Further, in order to perform this type of "partial" molding,
non-conventional molding equipment is required. The gate for
injecting the mold compound cannot be at the parting line of the
mold. The mold will thus have a cavity plate that needs to be
removed from the mold press to unload the package. (The mold would
be a three-part design with a cavity plate section that has to be
removed from the mold to remove the completed package.)
Molding an entire package body over a die mounted to a leadframe
("fully molded") usually requires that the leadframe has a
"dambar", namely a continuous ring of metal surrounding the body
that prevents the plastic from flowing out of the mold cavity
between the external leads of the leadframe. The dambar then has to
be removed to isolate individual leads before the package is
usable. With high pin count packages, the leads are often delicate
and spaced closely (fine pitch), resulting in the need for very
fine precision tooling for the trimming operation. This type of
tooling is also very expensive, which adds to the overall cost of
packaging.
Molding of plastic (or other suitable encapsulant) around the
leadframe also causes some leakage of the plastic onto the
leadframe (flashing). The flash then has to be removed in a
separate de-flashing (dejunking) step.
Attention is directed to commonly-owned U.S. Pat. No. 5,051,813,
entitled PLASTIC-PACKAGED SEMICONDUCTOR DEVICE HAVING LEAD SUPPORT
AND ALIGNMENT STRUCTURE, which discloses plastic packaging with and
without dambars, dejunking, etc.
In the main, hereinafter, molding where the mold gate is disposed
at the parting plane of the two mold halves is discussed, as most
pertinent to the present invention.
The following U.S. Pat. Nos. are cited of general interest in the
field of packaging (annotations in parentheses): 3,405,441
(hermetic sealing process using glass and metal lid on a ceramic
substrate); 3,909,838 (package formed by sealing two halves or
pre-molded body around a molded pill package bonded to a
leadframe); 4,143,456 (glob top sealing devices mounted on a
substrate); 4,264,917 (silicon substrate with glob top
encapsulation); 4,300,153 (TAB device with a substrate bonded to
the bottom of the die; glob top encapsulation); 4,330,790
(tape-mounted device encapsulated using a metal carrier and epoxy);
4,363,076 (flat TAB assembly); 4,507,675 (molded heatsink package);
4,594,770 (bonding a metal cap and a plastic cap around a
leadframe); 4,857,483 (mold gate is not located at the parting
plane of the mold halves); 4,872,825 (encapsulation method using a
lamination process instead of injection or transfer molding);
4,874,722 (pre-molded flatpack encapsulated with silicone gel;
dambar required; not encapsulated by molding); 4,890,152 (molded
pin grid array package; not a surface mount flatpack construction);
4,913,930 (coating and encapsulating a device in a reel-to-reel
format); 4,955,132 (flip chip mounting to a substrate); 4,961,105
(die back metallization); 4,974,057 (die coated with resin and then
molded); 4,975,765 (high density flatpack with edge connectors; not
a molded package); 4,982,265 (stackable TAB); 4,984,059 (leadframe
tips overlap the top of the die surface); 4,996,587 (thin,stackable
package); and 5,025,114 (leadframe construction resulting in
multilayer structure for plastic packages).
DISCLOSURE OF THE INVENTION
It is therefore an object of the present invention to provide an
improved molded packaging technique.
It is a further object of the present invention to provide an
improved technique for overmolding a semiconductor die on a
substrate.
It is another object of the present invention to provide a molded
packaging technique that is relatively inexpensive and relatively
easy to manufacture, and reliable.
It is another object of the present invention to provide a
technique for overmolding a semiconductor die on a substrate that
prevents de-lamination (detachment) of the molding material
(encapsulant) from the substrate.
It is another object of the present invention to provide a
technique for overmolding a semiconductor die on a substrate that
produces locating pins as a part of the molding process.
It is another object of the present invention to provide a
technique for overmolding a semiconductor die on a substrate that
produces standoffs (spacers) as a part of the molding process.
According to the invention, an overmolded semiconductor package
comprises a substantially planar substrate, a semiconductor die
mounted to the top surface of the substrate, one or more holes
disposed through the substrate, and molding compound disposed over
the top surface of the substrate, covering the semiconductor die
and extending through the one or more holes therethrough, thereby
forming rivet-like anchors on the bottom surface of the
substrate.
The anchors draw the molding compound on the top surface of the
substrate firmly to the substrate, preventing detachment
therefrom.
In one embodiment of the invention, the molding compound extends
only partially over the top surface of the substrate.
In another embodiment of the invention, the molding compound
extends completely over the top surface of the substrate.
In another embodiment of the invention, slits are disposed in the
molding compound, extending towards the top surface of the
substrate. The slits provide for some flexibility of the molding
compound (after curing) so that the molding compounds can conform
to some flexing of the substrate without cracking or delaminating
(detaching).
In one variation of this embodiment, the slits extend partially
through the molding compound towards the top surface of the
substrate. Another variation permits the slits to extend completely
through the molding compound, partially exposing a portion of the
top surface of the substrate underlying the molding compound.
In another embodiment of the invention, one or more tapered holes
are provided through the substrate, each having a dimension at the
bottom surface of the substrate greater than its corresponding
dimension at the top surface of the substrate, and one or more
rivet-like anchors are formed at the bottom surface of the
substrate through tapered holes by the molding compound.
In one variation of this embodiment, the rivet-like anchors formed
through the tapered holes are molded flush with the bottom surface
of the substrate.
The invention additionally provides for molded structures on the
bottom surface other than rivet-like anchors. These structures are
formed in the same manner by allowing the molding compound to flow
through holes in the substrate, then forming it on the other side
of the substrate.
In one such embodiment, at least one molded structure formed on the
bottom surface of the substrate includes a rivet-like anchor.
In another embodiment, at least one molded structure formed on the
bottom surface of the substrate includes a locating pin.
Other embodiments provide for molded structures formed into
standoffs and combination standoff/locating pins.
In another embodiment, a plurality of molded structures include
locating pins arranged in a pattern, where the pattern of the
locating pins is adapted to enforce a particular orientation of the
package when inserted into a set of corresponding holes formed in a
printed circuit board.
The present invention can also accommodate leadframe structures.
This embodiment of the invention provides a substantially planar
first substrate and a substantially planar second substrate having
a centrally located opening. A leadframe is sandwiched between the
bottom surface of the first substrate and the top surface of the
second substrate, such that the leads of the leadframe extend
outward beyond the two substrates. A semiconductor die is attached
to the bottom surface of the first substrate through the opening in
the second substrate, and one or more holes extend through the
first and second substrate from the top surface of the first
substrate to the bottom surface of the second substrate. An
overmolded "cover" (package body) is formed from molding compound
disposed over the semiconductor die, completely filling the
opening, partially covering the bottom surface of the second
substrate, and extending through the one or more holes to the top
surface of the first substrate, where one or more molded structures
are formed.
Various embodiments provide for similar molded structures and
arrangements to those described hereinabove.
Other objects, features and advantages of the invention will become
apparent in light of the following description thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a first embodiment of the
over-molded package of the present invention.
FIG. 2 is a cross-sectional view of a second embodiment of the
over-molded package of the present invention.
FIG. 3a is a cross-sectional view of one embodiment of a flush
rivet-like anchor according to the present invention.
FIG. 3b is a cross-sectional view of another embodiment of a flush
rivet-like anchor according to the present invention.
FIGS. 4a-4e are cross-sectional views of various embodiments of
molded locating pins, molded rivet-like anchors, molded standoffs,
and combinations thereof.
FIG. 5 is a view of the bottom side of a semiconductor package
illustrating the use of locating pins for keying, according to the
present invention.
FIGS. 6a and 6b are top views of two embodiments of molded
packages, according to the invention, illustrating the use of
molded slits.
FIG. 7 is a cross-sectional view of an alternate embodiment of a
molded package, according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a partially over-molded package 100, according to the
present invention. A semiconductor die 130 is mounted to an
upward-facing (as viewed in FIG. 1) surface 110a of a substantially
planar substrate 110. Holes (or slits) 112 and 114 are provided
through the substrate, as shown in the Figure. Pins 140 are
disposed in an array on a downward-facing surface 110b of substrate
110. It will be understood by one of ordinary skill in the art that
conductive traces extending from the pins 140 towards the die 130
provide electrical connection points by which electrical
connections may be established between the die 130 and the pins 140
(typically by bond wires, solder bumps, or other suitable
connection technique).
A molding compound 120 (preferably plastic, moldable epoxy, or
other similar suitable material) is formed into a "cover" or
overmolded package body 121 over the die 130 in an overmolding
process, and extends partially over the upward-facing surface of
the substrate 110, such that a small amount of molding compound 120
flows through the holes (or slits) 112 and 115, forming integral
(with the package body 121) rivet-like anchors 116 and 118.
Preferably, depressions are provided in a mold cavity coincident
with the holes (or slits) 112 and 114 which form the shapes of the
rivet-like anchors 116 and 118 during the overmolding process.
These rivet-like anchors, when cured, firmly secure the overmolded
package 121 to the upward-facing surface 110a of the substrate 110,
effectively limiting the ability of the molding compound 120 to
delaminate (detach) from the substrate 110.
While the rivet-like anchors 116 and 118 are shown as having a
generally rounded shape, these are merely exemplary of a suitable
shape. The invention places no specific requirement on the shape of
the rivet-like anchors.
In general, the molding process is the same for all embodiments of
the present invention. A mold cavity with two halves is provided.
The top half forms the shape of the overmolded top surface while
the bottom half has depressions which form the shapes of the
rivet-like anchors.
FIG. 2 shows a fully over-molded package 200, according to the
invention. In much the same manner as that shown with respect to
FIG. 1, a semiconductor die 230 is mounted to an upward-facing
surface 210a of a substrate 210, forming electrical connections
(not shown) with an array of ball bump contacts 240, disposed on a
downward-facing surface 210b of substrate 210. Holes 212 and 214
extend through the substrate 210. A molding compound 220 is formed
into an overmolded package body 221, completely covering the die
230 and the upward-facing surface 210a of substrate 210, and at
least partially around the ends of the substrate 210. In a manner
similar to that shown with respect to FIG. 1, an amount of molding
compound 220 flows through holes 212 and 214 to form rivet-like
anchors 216 and 218. These rivet-like anchors secure the overmolded
package body 221 to the upward-facing surface 210a of substrate 210
preventing delamination (detachment) therefrom.
In this embodiment, the rivet-like anchors 216 and 218 may be
formed such that during subsequent assembly of the package 200 to a
circuit board they act as spacers therebetween, thereby providing a
minimum distance to facilitate the formation of reliable bump
contacts of a pre-determined mechanical structure.
It will be readily appreciated by one of ordinary skill in the art
that the rivet like anchors (e.g., 116 and 118 with respect to FIG.
1, or 216 and 218 with respect to FIG. 2) may be disposed either
within or without the array of pins (140, FIG. 1) or bump contacts
(240, FIG. 2). FIG. 1 shows rivet like anchors 116 and 116 disposed
within the array of pins 140, and FIG. 2 shows rivet-like anchors
disposed without (outside of) the array of bump contacts 240.
According to the present invention, any of the features shown with
respect to FIGS. 1 and 2 may be interchanged, i.e., an embodiment
of the invention may be fully overmolded or partially overmolded;
may have an array of pin-type contact, bump contacts, or any other
suitable electrical connections; and may have rivet like anchors
disposed within (inside of) or without (outside of) the array of
electrical connections (e.g., pins, bump contacts, etc.).
In FIGS. 3a and 3b, and in the following discussion with respect
thereto, pins, bumps, and other electrical connections have been
omitted for illustrative clarity.
FIG. 3a is a cross-sectional view of a substrate 310, an upward
facing surface 310a of which is over-molded by a molding compound
320 forming an overmolded package body 321a, whereby a flush,
integral, rivet-like anchor 330a is formed in a manner similar to
that described hereinabove with respect to FIGS. 1 and 2. A hole
312 (analogous to the holes 112 and 212 described hereinabove with
respect to FIGS. 1 and 2) through the substrate has a flared
portion (chamfered edge) on a downward facing side 310b of the
substrate 310. During the molding process, the molding compound 320
is permitted to flow into the hole 312, but is prevented from
flowing out past the downward-facing side of the substrate 310 (by
a flush surface on the corresponding part of the mold cavity). The
flush, integral, rivet-like anchor 330a thus formed similarly
assists in preventing delamination (detachment) of the molding
compound 320 from the substrate 310, without extending out of the
downward facing surface 310b of the substrate 310.
FIG. 3b shows and embodiment similar in most regards to that shown
with respect to FIG. 3a. In this embodiment, a substrate 310' is
overmolded with a molding compound 320 to form an overmolded
package body 321b on an upward facing surface 310a' of the
substrate 310', simultaneously forming a flush, integral,
rivet-like anchor 330b in a manner similar to that described with
respect to FIG. 3a, except that the flush rivet-like anchor 330b is
formed in a tapered hole 316 (analogous to the holes 112, 212
described hereinabove with respect to FIGS. 1 and 2), rather than
in a straight hole with a flared portion. The tapered hole 316 has
walls which flare outward towards the downward facing surface 310b'
of substrate 310'.
It will be appreciated by one of ordinary skill in the art that any
opening through the substrate which has a dimension on the
downward-facing surface of the substrate greater than its dimension
on the upward-facing surface of the substrate is suitable for the
formation of a flush rivet-like anchor.
In general, holes provided through a substrate material, as
illustrated with respect to FIGS. 1, 2, 3a and 3b, permit the
formation of rivet-like anchors when overmolded with a suitable
molding compound. However, it is also possible to form the molding
compound which flows through the substrate into additional useful
structures by providing different shapes in the bottom mold cavity.
These are discussed hereinbelow with respect to FIGS. 4a-4e and
FIG. 5. As in FIGS. 3a,3b, pins/bump contacts are omitted in FIGS.
4a-e and 5 for illustrative clarity.
FIG. 4a is a cross-sectional view of a portion of an overmolded
package 400a. A top surface 410a of a substrate 410 is overmolded
by a molding compound 420 such that the molding compound 420 flows
through a hole 415 in the substrate 410 and is formed into a
locating pin 425 on the downward facing side (surface) 410b of the
substrate 410. This pin 425 extends into a hole 435 in a printed
circuit board 430, to which the package 400a is assembled,
providing a positioning reference therewith.
FIG. 4b is a cross-sectional view of a portion of another
overmolded package 400b, similar to the overmolded package 400a of
FIG. 4a, except that a rivet-like anchor 426 is additionally formed
on the downward-facing side 410b of the substrate 410. Package 400b
is similarly assembled to the printed circuit board 430, using
locating pin 425 as a positioning reference therewith.
FIG. 4c is a cross-section view of a portion of another embodiment
of an overmolded package 400c, similar to the embodiment 400a
described with respect to FIG. 4a. This time, the hole 415 through
the substrate 410 has a tapered (chamfered) portion 416 (see FIG.
3a for comparison). The molding compound is formed on the
downward-facing surface 410b of the substrate 410 such that
locating pin 425 extends from a flush rivet-like anchor formed in
the tapered portion of hole 415. As before, the locating pin 425
extends into hole 435, providing a positioning reference between
the package 400c and the printed circuit board 435.
FIG. 4d is a cross-sectional view of a portion of another
embodiment of an overmolded package 400d, similar to the embodiment
described with respect to FIG. 4b. In this embodiment, however,
instead of forming a rivet-like anchor 426, an integral
rivet/standoff structure 428 is formed, from which locating pin 425
extends. The rivet/standoff structure 428 is sized such that it is
greater than the width of the hole 435 and rests on one surface of
the printed circuit board 430 when assembled thereto. Locating pin
425 still provides a positioning reference, but the rivet/standoff
structure 428 provides a minimum gap between the circuit board 430
and the package 400d.
FIG. 4e is a cross-sectional view of still another embodiment of an
overmolded package 400e combining features of the packages
described with respect to FIG. 4c and 4d. In this embodiment, a
tapered portion 416 of hole 415 is provided, and a rivet/standoff
structure 429 is formed, this time without a locating pin (e.g.,
425). When the package 400e is assembled to the circuit board 430,
rivet/standoff structure 429 rests directly on the surface of the
circuit 430, providing a minimum spacing between the package 400e
and the circuit board 430. In this case, the molded rivet/standoff
structure 429 has no locating pin extending therefrom, and is
simply used as a spacer.
It will be readily appreciated by one of ordinary skill in the art
that the tapered portion 416 of hole 415 is not an essential part
of the embodiment described with respect to FIG. 4e, which may also
be implemented without such a tapered portion 416. However, FIG. 4e
demonstrates that the various features of the present invention may
be used alone or in combination. Further, where a plurality of
molded structures such as those described with respect to FIGS.
4a-4e are formed on a single package, they may be mixed, such as is
described hereinbelow with respect to FIG. 5.
FIG. 5 shows an overmolded package 500 of the type described
hereinabove, where the top surface of a substrate 510 has been
partially or fully overmolded, forming a plurality of molded
structures on the bottom surface of the substrate 510. At three
corners of the bottom surface of the substrate 510, molded
rivet/standoff structures 528 (see 428, FIG. 4d) are formed from
which locating pins 525 (comparable to locating pin 425 with
respect to FIG. 4a-d) extend. In a fourth corner, a rivet/standoff
structure 529 (see 429, FIG. 4e) is formed. The pattern formed by
the three locating pins 525 may be used to "key" the package 500 to
a printed circuit board, while the rivet/standoff structure 529 and
the integral standoffs 527 help to stabilize the package and
provide proper spacing between the package 500 and the circuit
board. That is, the pattern of the locating pins 525 is such that
they force a particular orientation of the package when inserted
into corresponding holes in the printed circuit board.
It will be readily appreciated by one of ordinary skill in the art
that several minor variations on this scheme are possible. For
example, FIG. 5 shows through a "missing pin" approach (i.e., all
of the molded structures are similar, but one has no locating pin).
Of course, any combination or number of standoff/locating pins and
standoffs may be used. Another variation on this theme varies the
shape or size of one or more of the locating pins, for example,
using a number of small locating pins and one large locating pin. A
similarly sized and positioned set of locating holes in a circuit
board effect the keying of the package to the circuit board.
Another variation on this theme uses one or more differently shaped
pins. For example, a triangular-shaped pin (and a mating triangular
shaped hole on the circuit board) may be used to effect the keying
of the package to the circuit board. Other shapes such as squares,
rectangles, other polygonal shapes, half-circles, pie sections,
etc., are equally effective.
While the various rivet-like anchors and combination structures
described hereinabove provide for better retention of the molding
compound, it is often necessary to provide for some additional
flexibility of the molding compound for larger packages. This is
accomplished by molding slits into the molding compound (on the
"die side" of the substrate) to permit some flexing of the molding
compound without cracking thereof. This is described hereinbelow
with respect to FIG. 6a and 6b.
FIG. 6a is a top view of a partially overmolded package 600a
according to the present invention. A substrate 610a is partially
overmolded with a molding compound 620a extending partially over
the top surface of the substrate 610a, and forming rivet-like
anchors 630 on the bottom surface of the substrate 610a by means
describe hereinabove. A plurality of molded-in slits 640 (three
indicated) extending in a number of different directions provide
for limited flexibility of the molding compound (after curing) such
that some bending of the substrate 610a may be accommodated without
cracking or delamination of the molding compound 620a. The slits
640 may extend either partially or completely through the molding
compound 620a. If they extend completely through the molding
compound 620a, then the top surface of the substrate 610a is
exposed.
FIG. 6b is a top view of a fully overmolded package 600b, identical
in all respects to the partially overmolded package 600a except
that substrate 610b is fully overmolded by molding compound 620b.
Rivet-like anchors 630 and molded-in slits 640 are similarly formed
and perform similarly.
The foregoing embodiments of the present invention have been
directed to planar substrates upon which (printed) conductive
traces are disposed. It is also possible, however, to apply the
techniques of the present invention to leadframe-type assemblies,
as described hereinbelow with respect to FIG. 7.
FIG. 7 shows an alternate embodiment 700 of a partially-molded form
of the present invention. In this embodiment, an upper PCB (or
substrate) 710 is formed as a simple planar substrate element,
without conductive lines (traces). A sandwich structure is formed
around a leadframe by upper PCB 710 and a lower PCB 714. Lower PCB
714 has an opening 716 adapted to receive a semiconductor die 730,
which is mounted directly to the underside 712 of the upper PCB
710. Leads 702 of the leadframe extend inward towards the opening
716. Holes 718 (one shown) are provided completely through upper
PCB 710 and lower PCB 714.
Whereas the upper PCB 710 does not have conductive lines, the lower
PCB is provided with a wiring layer of conductors (traces) 704.
These conductors 704 each extend from a respective through hole 706
towards the inner periphery of the lower PCB, preferably to within
0.010 inches of the opening 716 so as to be adjacent the die 730.
The conductors 704 are electrically connected to the through holes
706. A preferred method of making this connection is simply plating
the through holes 706. In essence, the through holes 706 are simply
formed as plated through vias, a well known technique. Inner ends
of the conductors 704 are left exposed, for connecting to the die
730 via bond wires 732. The remaining, outward portions of the
conductors 704 are preferably coated with solder mask material, for
purposes of electrical insulation.
A molded plastic (or epoxy, or other suitable material) encapsulant
720 is formed over the die 730 and a portion of lower PCB 714,
completely covering opening 716, bond wires 732 and the inner ends
of the conductors 704. The molding process is performed such that a
small amount of encapsulant 720 flows through holes 718 (one shown)
forming rivet-like anchors 726 (one shown). As in the other
embodiments, the rivet-like anchors 726 tend to prevent
delamination of the encapsulant.
Generally, this embodiment provides many of the same advantages and
benefits as the previous embodiments.
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